1. Field of the Invention
The present invention relates to a cam mechanism which is incorporated in a retractable zoom lens (lens barrel).
2. Description of the Related Art
The assignee of the present invention has proposed an advancing/retracting movement guide mechanism in U.S. Pat. No. 7,039,308, wherein the advancing/retracting movement guide mechanism has a structure making it possible to achieve a reduction in length of a cam ring in an optical axis direction without sacrificing the range of movement of each lens group or the like provided as a driven member in a lens barrel. This cam mechanism is characterized in that a plurality of cam grooves which trace substantially the same reference cam diagram are formed on the cam ring at different positions at least in the optical axis direction, that all cam grooves of the plurality of cam grooves are formed as partial cam grooves each having at least one end opening at least one of the opposite ends of the cam ring, so as not to include an entire portion of the reference cam diagram, that a plurality of cam followers are formed on a linearly movable ring at different positions at least in the optical axis direction to be respectively engageable in the cam grooves, and that at least one of the cam followers remains engaged in the associated cam groove while at least one of the other of the cam followers comes out of the end opening and is disengaged therefrom when the linearly movable frame moves to a movement limit thereof in the optical axis direction.
In a cam mechanism incorporated in a lens barrel, it is often the case that a plurality of cam grooves and a corresponding plurality of cam followers are provided at different positions in a circumferential direction about a photographing optical axis in order to secure stability. For example, if each of three cam grooves which are formed on a cam ring at three different positions in a circumferential direction is composed of two partial cam grooves, i.e., a front partial cam groove and a rear partial cam groove which trace substantially the same reference cam diagram by applying the feature of U.S. Pat. No. 7,039,308 to this cam mechanism, at least six partial cam grooves exist on the periphery of the cam ring. Additionally, six cam followers engageable in the six partial cam grooves are formed on the linearly movable ring. However, in the cam mechanism disclosed in U.S. Pat. No. 7,039,308, the possibility that a front cam follower and a rear cam follower that are provided as a pair of cam followers will simultaneously engage in a corresponding pair of (front and rear) partial cam grooves, respectively, is not excluded, and accordingly, a state where the six cam followers simultaneously engage in the six partial cam grooves occurs.
It is generally the case that it is more difficult to control the overall accuracy of the cam mechanism as the number of cam grooves and associated cam followers increases. Namely, in order to make the cam mechanism perform accurate camming operations, the accuracy of engagement between each cam groove and the associated cam follower only needs to be enhanced. However, if such accuracy of engagement is enhanced, the tolerance of the accuracy of engagement becomes small between another cam groove and the associated cam follower. On the other hand, in the cam mechanism disclosed in U.S. Pat. No. 7,039,308, the accuracy of engagement between each partial cam groove and the associated cam follower is required to be strictly precise so that a problem of inadmissible play, or the like, between the remaining cam followers and the associated cam grooves does not occur even in a state where some cam followers come out of the associated partial cam grooves. Namely, there are two contradictory demands: a demand for enhancing the accuracy of engagement between each cam groove and the associated cam follower and a demand for easing the accuracy control of each pair of front and rear cam followers and the associated pair of front and rear cam grooves.
Furthermore, in the above-described cam structure, it is desirable to further reduce the sliding friction between the cam followers and the cam grooves in order to reduce the rotational load on the cam ring.